To reaffirm that this issue is experienced due to the modification of server certificate by proxy service, the GlobalProtect logs file (PanGPS) can be searched for similar messages:
. try to connect to proxy, nProxyIP=964fa79d, proxyPort=8081, proto=6 . connect to proxy now . s=908, destName=1.2.3.4, nPort=443, nProxyIP=964fa79d, nProxyPort=8081, proxyuser=, proxypass= . Proxy connection established . Unable to verify server cert. Result is unable to get local issuer certificate . . Failed to verify server certificate of gateway example.com. . Show Gateway GP-External-Gateway: The server certificate is invalid. Please contact your IT administrator. . Failed to retrieve info for gateway example.com.